When a rhododendron does this, it is cold!
Usually at about this time of year my rhododendrons begin to communicate with me in a few different ways. One look out my window at the curled and drooping leaves tells me that it’s cold. How cold? That depends on the variety and the tightness of the curl and drop of the leaf. The best way to know is to watch your plant closely and compare the amount of leaf curl and droop to what the thermometer says. Do this for a winter season and soon you won’t need a thermometer at all, because your rhododendron will tell you all you need to know. There are several theories about why rhododendron leaves do this, but the most plausible (to me) is that the leaf curls to prevent moisture loss. Another theory that sounds likely is that the leaf is protecting its soft underside by curling it up inside the tougher, waxy outer surface. In any event this behavior doesn’t harm the plant, and once it warms up the leaves will perk up and flatten right out again.
Another way rhododendrons communicate is by bud growth, as the photos below show.
This large, fat, and kind of roundish bud that appears in the center of the whorl of leaves at the tip of a branch is a flower bud. Paying attention to how many of these are on the shrub will give a general idea of how many blossoms it will have for the following season. Usually there will be one large flower bud with 2 or 3 smaller leaf / branch buds around its base.
These smaller, slimmer and longer, more pointed buds also appear in the center of the whorl of leaves at the tip of a branch but are leaf/branch and secondary buds. There are usually 3-5 buds in a group with the center branch bud the largest of the group. The center bud will form a new branch with its own whorl of leaves in the spring and the secondary buds will stay as they are. If the larger center bud is removed some of the smaller secondary buds will grow into branches to make the plant bushier. This removing one bud to cause two or more to grow in its place is called “pinching.” More leaf / branch buds than flower buds means poor flowering for the following season, and could indicate a need for more fertilizer.
Another way a rhododendron will communicate is by leaf color. Leaves should be dark green; not olive green, yellow or green with yellow spots or edges. Yellow leaves could mean anything from an iron deficiency to soggy soil to soil with too high of a pH. Testing soil pH and checking drainage conditions would be the place to start investigating yellowing leaves, followed by a very light fertilizing in spring if the plant has been neglected. A spray of chelated iron will help with an iron deficiency.